This procedure is the most difficult described. Not because the reaction is difficult to perform, but because of the equipment one must build in order to make it workable. The problem is one of scale; A two-mole reduction can be performed in 5000ml glassware using a heating mantle and standard stirring equipment. To reduce a 20-mole batch requires ten times the volume, 50 liters, a controllable heat source, and scaled-up stirring equipment. Additionally, we will perform a steam-distillation in order to extract and purify the final product, which will require a large condenser. We will want this condenser to also operate in the reflux mode during the reaction in order to keep the acid from boiling off and killing everything. To make matters worse, we will be working with 15 liters of muriatic acid, which means everything must be made of stainless-steel. Fortunately, this equipment is neither difficult nor expensive to construct. One needs to either own and be proficient with a TIG welder or find a shop that can do the work. If one must use shops, spread the work around. They will all ask what it's for-simply tell them that you have a non-disclosure agreement and would lose your job or contract if you told them. Or make up your own story.
Our reaction vessel will be a 50-liter stainless stock-pot found at the kitchen/restaurant supply. It must be stainless, and not aluminum. High-quality stainless pots have aluminum-clad bottoms for better heat transfer-this is good. They will all be fabricated of thin-wall stainless, but look for the most heavy-duty pot you can find. This pot is the weak point in our equipment because the boiling hydrochloric acid will eat through the wall of the pot in 5-7 reactions, after which a new pot must be purchased.
In order to seal the pot, a flange must be welded onto the pot rim and provision made for attaching and sealing a top. This is done by carefully measuring the diameter of the pot rim and fabricating a flange to fit. We will be using a 3/16" polypropylene "O" ring for a seal and a series of bolts on the outside to accomplish the sealing and attaching. The ring should be about 2" wide, with the bolts(3/8" stainless hardware) on the outside and a 1/16" x 3/16" groove machined into the flange face about ¾" from the inside rim. Polypropylene O-ring material can be found at most good hydraulics shops and machine shops. We will be using 1/8" diameter or 3/16" diameter O-ring material, whichever is available. Buy enough for several rings, as they wear out. Our flange must also be flat to within 1/16" so the top isn't warped. Do not weld the flange to the pot until the top-plate has been fabricated.
We also have a requirement for knowing the temperature of the reaction at all times. In order to do this, one must either weld a stainless bushing into the side of the pot that fits an industrial thermometer or create a fitting in the top through which one can insert a thermometer long enough to reach the reaction solution. Good luck finding a thermometer that long. This writer chose the bushing-in-the-side method with mixed results-the temperature indication worked great but the boiling acid eats the thermometers and the bushing weld creates a weak point which the acid attacks and eats through after only three reactions. Covering the weld completely with fast-drying J-B Weld after each use doubled the lifespan of the pot to six reactions. I'm sure someone out there can think of something better. A Teflon- coated pot would be nice.
Now that we have the beginnings of a reaction vessel, we will need a heat source. Fortunately, one can find propane- powered barbeques almost anywhere, including the heavy-duty one pictured, which is more than adequate. A 30lb propane tank is good for about three reactions.
Now we must design our top-plate, which is fairly complex. The first step is to have a matching circle of 3/16" thick stainless-steel cut and drilled to fit the flange so they can be bolted together. Next, we must provide for a Teflon bearing in the middle. This writer designed a bearing machined from 3" Teflon round stock. Since the gearmotor used to turn the stirrer has a ½" driveshaft, the bearing consisted of a ½" hole in the middle and a ¾" wide outside shoulder machined down to ½" depth. This results in a ½" thick bushing wall, which has held up without problem. Using these dimensions, a center-hole of 1.5" diameter and four 10-32 threaded holes on the outside rim are required. It is advisable to have the Teflon bushing machined first and then fitted to the top. A shaft clearance of 0.003-5 works well. Unfortunately, one must know the diameter of the motor driveshaft in order to design the bearing. Many motors have 5/8" driveshafts which will operate perfectly with the above bushing design by simply enlarging the center hole, leaving a 3/8" bushing wall. The stirring shaft should be standard 316 stainless round stock with a flat machined at one end to make attaching the vanes eas